Which is why I don't believe in trading prospects/picks except in certain circumstances:
1) you're a legitimate playoff contender, not a bubble team but a top 8 or close to it team
2) you're trading low value assets for patches that will make you more competitive in the playoffs
3) you're trading surplus high value assets for top 9/top 4 players under control or with whom you're pretty sure you can negotiate a reasonable extension
So St Louis gave up 3 1st rd picks and some other assets to land ROR and Schenn, but both are in their prime and under team control for years.
It might hurt them 5 years from now, meanwhile they have enough young players to deal with it (Thomas, et al).
Islanders gave up a 1st and 2nd for Pageau, but he's locked up long-term and has been a key to getting to the CF. They got a iffy draft record anyway (Bellows, Wilde et al) so they got a sure thing for picks in the 20s and 50s.
When you're a top 8 or so team, you're trading #20+ in the draft, a valuable but not prime asset (though St Louis kinda screwed up when the Flyers got #14 and landed Farabee), so if you're getting a starter who'll play a major role, it's usually worth it.
Flyers have a deep prospect pool, so packaging a couple prospects and a 1st rd pick for a clear upgrade under team control makes sense at some point, but not for veteran "name" patches (i.e. 1 or 2 years left on the contract, no cap room to resign, or a player over 30 with a short shelf life).